High-temperature plastic flow in polycrystalline Al2O3 is often affected seriously by the doping of small amount of cations. For instance, a doping of either YO1.5 or ZrO2 in the level of 0.1mol% or less into Al2O3 increases very much the flow stress or drastically reduces the creep strain rate. Such a dopant effect is caused by grain boundary segregation of doped cation. The plastic flow in fine-grained, polycrystalline Al2O3 takes place mainly by grain boundary sliding or grain boundary diffusion. To make clear the effect of cation segregation on grain boundary sliding directly, we have examined the sliding behavior using undoped and Y3+-doped Al2O3 bicrystals with a random boundary at 1450degreesC under an applied stress of 15MPa. It was found that the grain boundary sliding rate was retarded in two orders of magnitude by Y3+-segregation in the random boundary. The result clearly indicates that the Y3+-segregation is an intrinsic effect to suppress the grain boundary sliding. The effect can be understood from the change in ionic bonding strength in the vicinity of grain boundaries with dopant segregation.